Optical systems are used in a variety of applications that require amplified light at a particular wavelength, such as optical communication networks, medical imaging, and atmospheric remote sensing. Amplified light is provided by a light source that includes an optical amplifier. An optical amplifier amplifies light by passing it through a gain medium. The gain medium is a material that increases the power of light by stimulated emission when supplied with energy. Where laser light is desired, the gain medium is positioned between a pair of mirrors known as an optical cavity. Input light resonates between the mirrors while being re-amplified by the gain medium until the lasing threshold is surpassed and laser light is produced.
A gain medium has a peak gain associated with a transition frequency of its constituent elements. Light having a wavelength at the peak gain is more readily and more robustly amplified than light at other wavelengths. Consequently, the lasing threshold is lowest at the peak gain.
Where an optical system requires a particular wavelength of amplified light, the light source may include a tunable optical filter. Amplified light of a selected wavelength is obtained by tuning the filter to that wavelength and sending the light into the gain medium with sufficient input power to achieve a desired output power. However, while providing light of a selected wavelength, tunable optical filters also emit a low background level of light across a broad spectrum of wavelengths. When the input power is high enough to successfully amplify a selected frequency not at peak gain, the input power of background light at the peak gain can surpass the lasing threshold, resulting in undesired lasing, i.e., parasitic lasing. This so-called parasitic lasing leaches energy from the system, creates spurious spectral peaks, adds noise to optical signals, and diminishes the power of amplified light at the selected wavelength.
As a consequence, the useful range of a tunable filter is limited. For existing light sources to be used effectively, the tunable optical filter must be kept within a narrow tuning range surrounding the peak gain of the optical amplifier. Thus, once a light source is deployed in an optical system, use of the entire system is restricted by the gain medium of the optical amplifier to a narrow range of wavelengths defined by a peak gain of the gain medium. A variety of optical applications in medicine, research, and communication require a range of wavelengths of light broader than existing optical systems can handle and performing these applications requires multiple optical systems, each built around its own gain medium.